The present invention relates to a gallium nitride compound semiconductor light-emitting device using a sapphire substrate as a support and having n- and p-type electrodes on both the top and bottom surfaces.
As light-emitting devices such as semiconductor lasers or LEDs, gallium nitride compound semiconductor light-emitting devices using a sapphire substrate are used. In light-emitting devices of this sort, since a sapphire substrate is insulating and it is difficult to form holes in the substrate, n- and p-type electrodes are formed on the same surface. Consequently, the chip size is increased, and this decreases the number of chips obtainable from one wafer and increases the cost. Also, since the electrode gap cannot be narrowed, the device resistance increases. Additionally, the light emission efficiency is decreased because uniform carrier injection becomes difficult to perform. Furthermore, if the electrode gap is narrowed, an electric short circuit may occur.
To solve these problems, Jpn. Pat. Appln. KOKAI Publication Nos. 7-202325 and 7-221347 have disclosed light-emitting devices in which n- and p-type electrodes are formed on both the top and bottom surfaces of a gallium nitride compound semiconductor multiple layer stacked on a sapphire substrate. In the fabrication of the light-emitting devices disclosed in these references, a gallium nitride compound semiconductor multiple layer having n- and p-type layers is stacked on a sapphire substrate. Subsequently, a portion of the sapphire substrate is removed from the bottom surface to expose the bottom surface of the semiconductor multiple layer. Thereafter, n- and p-type electrodes are formed on the top and bottom surfaces of the semiconductor multiple layer.
That is, in the light-emitting device fabrication methods disclosed in the above patent publications, a gallium nitride compound semiconductor multiple layer is stacked on a sapphire substrate, and then holes for forming electrodes are formed in the sapphire substrate. These holes are formed by means of a physical processing technique using, e.g., a dicer, or an etching technique such as dry etching or wet etching. However, these techniques have the following problems.
When the physical processing technique such as a dicer is used, it is essentially impossible to cut only the sapphire substrate with a high controllability without giving any damages to the stacked gallium nitride semiconductor layers.
When sapphire is to be processed by dry etching, it is necessary to use ion energy larger than ion energy when a gallium nitride semiconductor is etched. This increases the contribution of physical sputtering and decreases the etching selectivity between the sapphire and the gallium nitride semiconductor. As a result, it becomes difficult to stop etching with a high controllability at the interface between the sapphire substrate and the gallium nitride semiconductor layers. This gives etching damages to the gallium nitride semiconductor layers.
Also, the etching rate of sapphire is very low in dry etching or wet etching. For this reason, it is practically difficult to form holes in a sapphire substrate usually having a thickness of 350 .mu.m or more without damaging gallium nitride semiconductor layers.
As described above, the conventional techniques give damages to gallium nitride semiconductor layers when processing a sapphire substrate. This leads to serious problems of a decrease in the yield of devices and degradation of the device characteristics.